(i) Field of the invention
This invention relates to a process for the membrane separation of gases, and more particularly to the separation of gaseous hydrides or mixtures of gaseous hydrides, such as SiH.sub.4, Si.sub.2 H.sub.6, AsH.sub.3, PH.sub.3, or even GeH.sub.4, frequently used in the microelectronics industry for operations such as the manufacture of metallic silicon, the deposition of silica SiO.sub.2, or even the doping of predetermined zones of a semiconductor.
(ii) Description of Related Art
The use of atmospheres of this kind comprising gaseous hydrides traditionally poses problems associated with:
a) the strict safety conditions under which these atmospheres must be used. The majority of them exhibit high risks of toxicity, to which can be added risks associated with the flammable nature of some of them. In this context, it is often recommended to use them within a certain safety or concentration limit in a carrier gas (this limit varying from one hydride to another), the difficulty being that the applications mentioned hereinabove generally require higher concentrations than this safety limit, as they sometimes reach several tens of %, and PA1 b)their high cost, urging users to attempt as far as possible to recycle the hydride or the mixture of hydrides not having reacted in the course of the process in question using this hydride or these hydrides. PA1 the purification of the mixtures obtained at the outlet of generators for the production of gaseous hydride, which, in addition to the hydride desired, produces residual species, such as hydrogen, water vapour, etc. An example is an electrolytic arsine generator producing a mixture of arsine and hydrogen with a higher or lower arsine concentration at the cathode; PA1 the treatment of the mixtures obtained at the outlet of reactors using hydrides or mixtures of hydrides of this kind: the separation of the hydrides not having reacted and the recycling of these hydrides back to the inlet of the reactor. An example in this area is that of reactors for the manufacture of metallic silicon from gaseous silane SiH.sub.4, and PA1 the safety concentration of hydrides in a carrier gas: in order only to use and convey sources (usually bottles) of gaseous hydrides including a hydride concentration in a carrier gas (a neutral gas or even hydrogen) lower than the safety limit mentioned hereinabove (e.g. 2% silane), and only to concentrate the hydride or the mixture of hydrides just upstream of the point of use where a high hydride concentration is required.
In such a context, the advantage of being able to separate hydrides of this kind will be clear, with aims such as:
The existing processes for the separation of gaseous hydrides from a gaseous medium are generally very costly and present real dangers from the point of view of handling, be it separation by adsorption or by cryogenic distillation.
Nevertheless, the recent studies of document U.S. Pat. No. 4,941,893 can be cited, proposing the separation of gaseous silicon compounds (SiX.sub.a H.sub.b, including silane) from hydrogen or halogenated acids (HX, X: Cl, Br, I, F) with the aid of a semi-permeable membrane, this document particularly recommending and illustrating the use of a sulphonated polysulphone/polysulphone composite membrane. The results obtained in the case of silane are such that, for an initial H.sub.2 /SiH.sub.4 mixture with 48.8% silane, the silane concentration at the membrane outlet varies from approximately 52% to approximately 84% according to the throughput in question at the inlet of the membrane. The concentration at the outlet varies between approximately 1.5% and 25% in the case of a silane concentration at the inlet of 1%.
The document U.S. Pat. No. 4,957,513 can also be mentioned, illustrating the membrane separation of hydrogen selenide (H.sub.2 Se) from a gaseous mixture containing in addition to H.sub.2 Se, H.sub.2 O and HCl, as obtained at the outlet of a hydrogen selenide generator. In view of the membranes used and the permeation properties of H.sub.2 Se in these membranes, the mixture enriched with hydrogen selenide is obtained at the permeate side of the membrane.